Solar cells are commonly fabricated from p-type crystalline silicon (c-Si) doped with boron. Such solar cells suffer from light-induced degradation (LID) that lowers the efficiency of the solar cells. This is generally ascribed to boron-oxygen (B—O) defects in the silicon.
One approach to reversing such LID involves a post-process step in which finished solar cells are subjected to a low-temperature thermal annealing step in which the solar cells are heated to a temperature between 50° C. to 230° C. while simultaneously generating excess carriers in the silicon. With this approach to reversing LID, excess carriers are generated in the silicon by either applying an external voltage to the solar cells or by subjecting the solar cells to illumination while the solar cells are heated. This LID-reversal approach is described in the patent application WO 2007/107351.
However, this approach for reversing LID is a post-process step, which is performed after fabrication of the solar cell would otherwise be complete. Using such a post-process step adds additional equipment and processing to the fabrication of solar cells. Also, this LID-reversal approach is performed at a low temperature so as to avoid damaging the solar cell that would result from higher-temperature heating.
It has been suggested that a similar approach can be used in the middle of solar cell fabrication during the step of passivation of defects in the silicon by hydrogenation. Using such an approach during hydrogenation passivation is described in the patent application WO 2013/173867. With this approach, the silicon wafer is illuminated during both the heating of the wafer and the subsequent cool down of the wafer during hydrogenation passivation. Such illumination during both heating and subsequent cool down can be performed during any subsequent thermal processes that may degrade the quality of hydrogenation passivation.
However, this approach for protecting the quality of hydrogenation passivation still typically involves illuminating the silicon while the silicon is being heated. Such an approach may not be suitable for use in high-performance, highly efficient solar cell fabrication lines that require precise and stable thermal profiles during metallization firing. Also, such an approach typically involves illuminating the silicon in the presence of a hydrogen source during cooling, which necessitates including a hydrogen source in the cooling chamber.